Specifications

Core Practicals

Electrolytic Processes

Electrolysis

Electrolysis is the process of passing electrical current (direct current) through a solution, or molten ionic compound, to decompose electrolytes. This separates the ions, and elements will be discharged at the electrodes. Inert electrodes should be used, otherwise they will react with the solution /products.

  

Electrolysis can only be done using ionic compounds, as it requires free ions to move. This is why it can only be done if in solution, or liquid (molten) state. If electrolysis is completed with a solution, then hydrogen (H⁺) and hydroxide (OH⁻) ions will also be present, as water will also dissociate.

  

During electrolysis:

  • positively charged ions (cations) migrate to the negatively charged electrode (cathode)

  • negatively charged ions (anions) migrate to the positively charged electrode (anode)

31 electrolysis-01.png

Electrolysis of Solution

You need to be able to explain the formation of products during specific electrolysis experiments, when inert electrodes are used.

  

In the table below, we can see which products are formed. If you are unsure why the following discharges happen, look again below at the rules of electrolysis.

Rules of electrolysis

  • at the cathode (negative electrode) - the least reactive element (metal or hydrogen) will be discharged

  • at the anode (positive electrode) - in order of preference:

    1. halide ions (F⁻, Cl⁻, Br⁻ etc.) will be discharged as halogen gases

    2. hydroxide ions (OH⁻) - given off as oxygen gas

    3. any other negative ions present

32 solutions-01.png

Half Equations (Higher Only)

half equation shows what happens at each electrode during electrolysis, in terms of the electron transfer. It shows what happens when ions gain electrons (reduction) or lose electrons (oxidation). In half equations:

  • electrons are shown as e⁻

  • the numbers of atoms or ions of each element must be the same on both sides (like in a balanced equation)

  • the total charge on each side needs to be the same

At the anode

Negative ions (anions) will gather at the anode, and lose electrons. You will either need to write a half equation for a halide, oxygen, hydroxide or a metal.

2Cl⁻(aq) - 2e⁻ → Cl₂(g)
2O²⁻(aq) - 4e⁻ → O₂(g)
4OH⁻(aq) - 4e⁻ → 2H₂O(l) + O₂(g)
Cu(s) - 2e⁻ → Cu²⁺(aq)

At the cathode

Positive ions (cations) will gather at the cathode, and gain electrons. You will either need to write a half equation for a metal, or hydrogen gaining electrons. 

  

Group one metals, and hydrogen, only have a +1 charge, and so only need to gain one electron, but notice how because hydrogen gas is made of two hydrogen atoms, we must start with 2 hydrogen ions:

2H⁺(aq) + 2e⁻ → H₂(g)

  

Other cathode half equations include:

Na⁺(aq) + e⁻ → Na(s)
K⁺(aq) + e⁻ → K(s)
Cu²⁺(aq) + 2e⁻ → Cu(s)

Electrolysis of Copper

Purifying copper

Copper can be purified by electrolysis. Electricity is passed through solutions containing copper compounds, such as copper sulfate. In this process, the anode would be made from impure copper and the cathode made from pure copper.

  

Copper ions are positive (Cu²⁺), so move to the negative electrode (cathode). During electrolysis, the anode loses mass as copper dissolves (as the ions are moving to the cathode), and the cathode gains mass as copper is deposited. A sludge will form under the anode containing impurities. This shouldn't be thrown away as may contain expensive metals like silver.

  

Electrolysis of copper sulfate with graphite electrodes

Using inert electrodes (such as graphite electrodes) will still produce copper deposits at the cathode. However, rather than the anode mass decreasing (like it does in copper purification) we will produce oxygen gas. This can be tested for using a glowing splint, and when it relights - it proves that oxygen is present.

33 copper electrolysis-01.png

Core Practical 4: Electrolysis of Copper Sulfate Solution

This involves setting up an electrolysis to investigate the effect of changing the current on the mass of the copper electrodes used in the electrolysis of copper sulfate solution.

  

The second part of this investigation covers the products formed during the electrolysis of copper sulfate solution using inert (graphite) electrodes. Quantitative analysis when using copper electrodes will be expected.

copper electrolysis-02.png

Part 1 - Investigation using inert electrodes

  1. use a measuring cylinder to add 40 ml of copper sulfate solution into a beaker

  2. place two graphite rods into the copper sulfate solution - attaching one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal

  3. fill a small test tube with copper sulfate solution and position over the positive electrode (anode) - as shown in the diagram above

  4. turn on the power supply and observe what happens at each electrode

  5. test any gas produced with a glowing splint

  6. record your observations and the results of your tests

Part 2 - Investigation using copper electrodes

  1. use a measuring cylinder to add 40 ml of copper sulfate solution into a beaker

  2. measure and record the masses of two pieces of copper foil, labelling one the anode, and one the cathode

  3. attaching the cathode to the negative terminal of a dc supply, and the anode to the positive terminal

  4. turn on the power supply, adjust the power supply to achieve a constant current as directed by your teacher, and observe what happens at each electrode

  5. after 10 minutes, turn off the power

  6. carefully remove the electrodes and allow all the liquid to evaporate - do not wipe the electrodes clean

  7. measure and record the mass of the electrodes

  8. repeat the experiment again with new electrodes, and different currents

 
Get in touch

© revise-science.uk 2020

London, United Kingdom

Terms and Conditions | Privacy Policy

info@revisechemistry.uk